The present invention relates to ceramic materials for use in nuclear fusion reactors and more particularly to coated ceramic breeder materials which exhibit high thermal conductivity while concurrently functioning as a neutron multiplier.
Tritium (H.sup.3), an isotope of hydrogen, has been suggested as a fuel to produce energy in a thermal nuclear reaction in accordance with the following equation. EQU D+T.music-sharp.He.sup.4 +n+17.6 mev.
In this reaction the deuterium nucleus (D) undergoes fusion with a tritium nucleus (T) to produce a helium--4 nucleus and a neutron (n) with the release of energy. This energy may be converted to heat energy through collision with and absorption by heat transfer materials surrounding the reaction vessel. The heat transfer materials provide a through path for heat transfer fluid such as helium and contains a breeder blanket assembly which also acts as a tritium breeder.
In general, fusion reactor breeder materials which have been used in the recovery of tritium from a lithium containing ceramic breeder blanket are Li.sub.2 O, .gamma.LiAlO.sub.2, Li.sub.4 SiO.sub.4 and Li.sub.2 ZrO.sub.4. Other lithium containing ceramic materials have also been used. Different problems have been observed in the use of these materials such as fracturing or striation of the materials, sometimes resulting in material shifting and rearrangement within the blanket. Such fractures are due primarily to thermal stresses, and reduce the thermal conductivity of the blanket materials and thus the efficiency and temperature distribution of the blanket.
The use of such lithium containing ceramics usually requires a neutron multiplier, such as Be or BeO, which is generally introduced as a dispersion of Be or BeO amongst the lithium containing ceramic materials.
Two shapes or configurations of particles in the breeder blanket for solid breeders have been suggested for fusion applications. They are: (1) pressed and sintered pellets and (2) sphere-pacC (spherical particles). The pressed and sintered technique has been used successfully for preparing Li.sub.2 O, Li.sub.4 SiO.sub.4, .gamma.-LiAlO.sub.2, and Li.sub.2 ZrO.sub.3. Technology in the formation of spherical particles has been developed for fission fuels and can be considered an attractive alternative for use in the fabrication of ceramic tritium breeder materials. A major advantage of the sphere-pac configuration over pressed and sintered materials is the ability of the sphere-pac configuration to significantly reduce the likelihood of fracture due to thermal stresses and increase contact with the container materials for better heat transfer. This results in enhanced mechanical and thermal stabilities of the solid breeder component.
As stated above, lithium containing ceramic breeding materials in sintered pellet (or block) form have been observed to fracture at a relatively low stress level with a subsequent degradation in thermal conductivity. While the sphere-pac configuration provides a solution to the fracture problem, the nature of the spherical particles is such that the thermal conductivity of the material becomes the dominant factor when considering the effectiveness of materials in such a form, primarily because of the limited contact between spheres.